a) Field of the Invention
The invention is directed to a method and an arrangement for plasma-based generation of intensive short-wavelength radiation in which a target flow comprising defined portions which is made available in a reproducible manner interacts with a pulsed energy beam for exciting radiation-emitting plasma. The invention is particularly suitable for the generation of soft x-radiation, preferably EUV radiation, for the exposure of very small structures in semiconductor lithography.
b) Description of the Related Art
In the prior art relating to energy beam pumped plasma-based radiation sources, mass-limited targets for plasma generation have become increasingly accepted because they minimize unwanted particle emission (debris) compared to other types of targets. A mass-limited target is wherein the particle number in the focus of an energy beam is limited to the order of magnitude of the ions used for generating radiation. In this connection, EP 0 186 491 B1 describes the excitation of individual droplets, i.e., exactly one droplet is impinged upon per energy pulse. The droplet size is of the same order of magnitude as the laser focus. When a pulsed energy beam is directed on a series of individual droplets, it is necessary to synchronize both events with one another with respect to space and time. However, the generation of droplets in a vacuum chamber depends upon the characteristics of the target material and is not possible for every target material. For example, xenon cannot be used to generate individual droplets under the process conditions of EUV lithography.
Further, targets in the form of clusters (U.S. Pat. No. 5,577,092), gas puffs (H. Fiedorowicz, SPIE Proceedings, Vol. 4688, 619) or aerosols (WO 01/30122) have been described for plasma generation. However, the average density of such targets in the focus volume is substantially less than in liquid targets or solid targets because the target comprises microscopic particles or is in gaseous form. Further, the target divergence is generally so great (opening angle of several degrees) that the average target density decreases rapidly with increasing distance from the nozzle. Therefore, the energy beam can be coupled in efficiently only in the immediate vicinity of the nozzle, which leads to a high thermal loading of the nozzle and inevitably results in nozzle erosion.
While arrangements with a continuous target jet (liquid or frozen jet) such as is described in WO97/40650, for example, allow a relatively large working distance from the nozzle, they are susceptible to shock waves. This means that the radiation-generating energy pulse that is coupled in causes hydrodynamic disturbances extending relatively far along the jet axis and the characteristics of the continuing jet for optimal plasma generation and radiation generation are impaired. These disturbances prevent a high pulse repetition frequency because it is necessary to wait for the disturbances to die down before the next pulse.
Further, the prior art includes plasma-based radiation sources with optimized energy conversion in a determined spectral range. For example, it is known from U.S. Pat. No. 4,058,486 to use a defined pre-pulse prior in time to a main excitation pulse generating the radiating plasma in order to increase the efficiency of the energy conversion of the excitation radiation into emitted radiation of the plasma in the EUV range.